Mapping the Status of Desert Streams

Many people in the arid Southwest care about the fate of perennial streams and their associated riparian communities. The loss of flows in streams and rivers has social, economic, and ecological consequences, so managers and concerned citizens seek ways to track their status.

TNC scientist Holly Richter monitors the San Pedro River from her horse. Photo credit: Kelly Tighe

In 1999, the Nature Conservancy launched a volunteer program involving scientists, managers, and citizen volunteers in southern Arizona who participated in a low-cost, comprehensive, and enjoyable method for monitoring our rivers—wet/dry mapping.

Every year on the third Saturday in June, people walk or ride horses along desert streams to map where the streams have surface flow and where they are dry. With more than a decade’s worth of data, this work is helping scientists and managers better understand and manage our riparian and aquatic habitats. The data have been consistently collected at the end of the dry summer months, right before the monsoon rains typically begin.

Wet/dry mapping uses citizen scientists to map the extent of surface flow in a river where certain reaches dry up during the summer. It provides a snapshot of conditions along the entire river at the same date each year, allowing comparisons of year-to-year variability.

Applications of wet/dry mapping

Wet/dry mapping has been used to:

quantify long-term trends in surface water patterns

better understand groundwater/surface water interactions

identify study reaches to design and implement ecological research and monitoring

manage fish and wildlife populations and riparian habitat

Wet/dry mapping along the San Pedro River

We have conducted wet/dry mapping for more than a decade on the San Pedro River and its tributary streams. In 2010, the effort involved 125 people on foot, horseback, and kayak, covering more than 220 stream miles. Partners include the Bureau of Land Management, Community Watershed Alliance, Friends of the San Pedro River, Cascabel Volunteers, CONANP, BIDA, Reserva Forestal Nacional y Refugio de Fauna Silvestre Ajos-Bavispe, and Naturalia.

2010 Surface San Pedro River surface water extent, based on wet/dry mapping. Click map to enlarge

Data from the first twelve years of wet/dry mapping in the San Pedro Riparian National Conservation Area (SPRNCA) reveal that wetted length varies from year to year, but the river hasn’t significantly changed overall. That’s good news, and suggests that conservation efforts by public and private stakeholders have made a difference. Indeed, a five-mile section of the river shows a steady increase in its wetted length where irrigated farm fields were retired for water conservation.

The length of wetted channel through the SPRNCA correlates with the volume of flow at the USGS Charleston stream gauge on the day of each wet/dry survey, validating a strong relationship between these approaches. However, the two monitoring methods tell different, but complimentary stories. Stream gauges provide an understanding of hourly changes throughout the year at a fixed point on the stream, while wet/dry mapping helps us grasp landscape-scale changes throughout the entire river on a fixed date.

Wet/dry mapping on other rivers

Cienega Creek wet/dry data from 2006-2010. Click map to enlarge

The wet/dry mapping approach has been applied in other stream systems around Arizona, including Agua Fria River (Arizona NEMO), and Cienega Creek (conducted by TNC, BLM, and Pima County), and tributaries of the San Pedro River such as the Babocomari River, Ramsey, Miller, Hot Springs, and Redfield Canyons, and Los Fresnos in Mexico.

Wet/dry mapping conducted since 2006 along Cienega Creek has revealed locations of previously-unmapped perennial flow. While locals knew better, official state maps did not recognize a major portion of Upper Cienega Creek, and two flowing reaches of its tributary, Empire Creek, as perennial. These locations were subsequently omitted from mapping efforts such as Arizona Game and Fish Department's statewide inventory of riparian vegetation in the mid-1990's. Further downstream where the creek has been tracked since 1984, once-reliable reaches now go dry in some years.

The value of citizen science

Wet/dry mapping turns a walk through the cottonwood forest into meaningful science. Participants in the past have included ranchers, realtors, regulatory agencies, environmentalists, City Councilmen, children, and reporters. It gives interested citizens a chance to learn more about their rivers and get their feet wet in the ecosystem. Along the way, mapping volunteers have encountered species such as coatimundi, mountain lion, leopard frogs, bear, Gila monsters, bobcats, gray hawks, and longfin dace.

Mapping methods

Wet/dry mapping is simple. Using hand-held GPS units, people record the start- and end-points of every wet portion along a stream or river. Using GIS software, these points are translated to lines on a map for display and analysis. To learn more, download detailed instructions and sample data forms for conducting wet/dry mapping.

Data release

The streams and rivers covered by wet/dry mapping flow through a mix of public and private lands. To respect the concerns of property owners, we do not release the wet/dry field data or GIS files for private lands. We will share GIS files for public lands, upon request.